This invention relates generally to ultrafine powders and, more specifically, to a method for synthesizing ultrafine powders using microwaves. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Recently many methods for manufacturing fine metal powders have been proposed. Efforts to develop these methods by industry are a result of the need for materials which are easily sintered and which yield products with improved mechanical properties.
U.S. Pat. No. 4,556,416 describes one process for manufacturing a powder. To use this process a reaction chamber is first exhausted to about 10.sup.-4 torr. A volatile metal compound gas or an evaporated metal or both and a reaction gas are mixed and supplied to the reaction chamber. The gas is then ionized by means of a high frequency induction coil. Simultaneously, the gas is irradiated by a laser. The reaction product and exhaust gas are discharged through an exhaust port and powder is collected by a filter.
This type of system has the disadvantage of having high energy consumption and high capital equipment costs because of the need for evacuation equipment, induction coils, and a laser. The invention also has the disadvantage of requiring a carrier gas such as nitrogen or hydrogen.
Conventional radio-frequency (rf) plasma tubes are used to produce fine powders but consume a lot of energy because the plasma is maintained at a temperature of about 10,000 K. Usually the starting material is in the form of an aerosol or liquid which is introduced into a gaseous plasma. Fine powders are then collected at the bottom of the rf plasma column. Large quantities of powders cannot be produced using conventional plasma tubes.
Another method of producing ultrafine powders is a mechanical technique whereby powders are ground by a vibration ball mill in a methanol medium. Using this technique 0.1 .mu.m diameter particles are manufactured only after 120 hours of grinding.
There are still other methods of making powders including evaporative decomposition of solids, hydrothermal processing, rapid expansion of supercritical fluids, and reductive dehalogenation of elemental halides. These methods and the disadvantages of each are discussed in L. M. Sheppard, "Low-Temperature Synthesis Of Ceramics," Adv. Mater. & Processes, Inc. Metal Prog., 47 (November 1986).
It is therefore an object of the invention to produce fine powders in a system which minimizes energy consumption.
It is another object of the invention to produce fine powders quickly and without the need of expensive capital equipment.